The problem is with apparmor, not with cups or the virtual printer driver.
For any who want the convenience of 'one-touch' printing to pdf, the package can be installed by its older name:
sudo apt install cups-pdf
Or by the newer:
sudo apt install printer-driver-cups-pdf
Then to change the default output file destination, the config file may be edited:
sudo gedit /etc/cups/cups-pdf.conf (Use gksu or gksudo in place of sudo if preferred.)
On the line:
Change the folder location as desired. Examples:
(If pasting an example line, use only one, like the original file.) Highlight the location part after the
$, copy, then save the file and close it. Next, open the apparmor config file:
sudo gedit /etc/apparmor.d/usr.sbin.cupsd
Find the two lines near the bottom with the old default path,
On each line, highlight everything between the
@ and the trailing
/ and paste the new dirspec just copied. Inspect to be sure only the two paths were changed, and there are no missing or extra characters or spaces inserted. (Make a backup copy beforehand if unsure, and remember ctrlz undo is your friend.) Ex.
Save, close and reboot, then test print. Should work.